It is true that the image itself is sufficiently permanent, and cannot be said, in correct language, to need _fixing_; but the unchanged Silver Salt which surrounds it, being still sensitive to light, tends to be decomposed in its turn, and so the picture is lost. It is therefore necessary to remove this salt by applying some chemical agent capable of dissolving it. The list of solvents of Chloride and Iodide of Silver has been given in Chapter II., but some are better adapted for fixing than others. In order that any body may be employed with success as a fixing agent, it is required not only that it should dissolve unchanged Chloride or Iodide of Silver, but that it should produce no injurious effect upon the same salts reduced by light. This _solvent action upon the image_, as well as upon the parts which surround it, is most liable to happen when the agency of light alone, without a developer, has been employed. In that case the darkened surface, not being reduced perfectly to the metallic state, remains soluble to a certain extent in the fixing liquid. CHEMISTRY OF THE VARIOUS FIXING AGENTS. The following will be mentioned:--Ammonia--Alkaline Chlorides--Alkaline Iodides--Alkaline Hyposulphite--Alkaline Cyanides. AMMONIA. The properties of the alkaline liquid "Ammonia" are given in Part III. Ammonia dissolves Chloride of Silver readily, but not Iodide of Silver: hence its use is necessarily confined to the paper proofs upon Chloride of Silver. Even these however cannot advantageously be fixed in Ammonia unless a deposit of Gold has been previously produced upon the surface by a process of "toning," presently to be explained: a peculiar and unpleasant red tint is always caused by Ammonia acting upon the darkened material of a sun picture as it comes from the printing-frame: but this is obviated by the employment of the Gold. ALKALINE CHLORIDES, IODIDES, AND BROMIDES. The Chlorides of Potassium, Ammonium, and Sodium possess the property of dissolving a small portion of Chloride of Silver. In the act of solution a double salt is formed; that is, a compound of Chloride of Sodium with Chloride of Silver, which may be crystallized out by allowing the liquid to evaporate spontaneously. The earlier Photographers employed a saturated solution of common Salt for fixing paper prints; but the fixing action of the Alkaline Chlorides is slow and imperfect, and their use may now be said to be obsolete. The Iodide and Bromide of Potassium have both been used as fixing agents. They dissolve Iodide of Silver, forming with it a double salt in the manner before described.